Drawboring increases the strength of a regular mortise-and-tenon joint by driving a wood pin through a hole in the mortised piece and through a slightly offset hole in the tenon.
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Photo of hands assembling a drawbore joint

Drawboring increases the strength of a regular mortise-and-tenon joint by driving a wood pin through a hole in the mortised piece and through a slightly offset hole in the tenon, above. This draws the shoulder of the tenon tight against the face of the mortise. The pin provides additional mechanical strength that keeps the joint tight, even when your joinery isn't a perfect friction fit. Assembly of the joint doesn't require clamps, which makes drawboring particularly attractive when dealing with large assemblies or oddly shaped pieces. 

Photo of table legs with drawbore joint
The centered legs on this table base create levers of the feet and top rails, amplifying stress on the mortise-and-tenon joints. Drawboring reinforces the joints and adds visual appeal.

You'll often find drawbore joints in large, timber-style-framed workbenches, and furniture assemblies that experience a lot of racking, such as the rails and feet of a trestle table (photo, above.) or the crest rail of a rocking chair. When finished, the driven pin reveals the beauty of contrasting wood grain that accentuates the forethought taken by the craftsperson to ensure a lifetime of use. Creating this joint requires just a few simple steps, as I'll show.  

The hole story

Section view drawing of drawbore joint
Drawbore Joint (section view)

Form your mortises and tenons however you prefer: on the tablesaw, drill press, router table, or by hand. Once you have a fitted joint (remember, it doesn't need to be a perfect friction fit), bore a hole centered on the depth of the mortise. (I drilled a 3⁄8" hole.) You can place the hole closer to the workpiece edge, if you like, as long as you allow sufficient material between the edge and the hole to prevent blow-out while driving the pin. There's no need to drill all the way through the bottom mortise wall (drawing, above), but you can if you want the pin visible on both faces. For a beefy assembly, such as the one shown, I typically drill just deep enough to bury the end of the pin in the far side of the mortise.

Photo of punch in hole of project
Place an undersize centerpunch against the wall of the hole closest to the tenon shoulder. Tap the punch to leave a dimple.

Next, dry-fit the joint to locate the offset hole in the tenon, using a centerpunch (or a brad-point drill bit) 1⁄16" smaller than the hole in the mortise (photo, above.). Disassemble the joint and drill on the mark a hole the same diameter as the one in the mortise. Use a backer piece to prevent blow-out as the drill bit exits the tenon. With the joint dry-fit, you can see the offset holes (photo, below.)

Photo of correctly bored hole
When correctly drilled, you see the hole in the tenon offset from those in the mortise.

Let's put a pin in this

The pins used to drawbore a joint must be flexible, but strong. This requires a straight-grained and stable hardwood—something difficult to find in purchased dowels. Instead, I make my own using scrap stock and a dowel former, a tool with an internal blade that shears the riven wood as you pound it through. 

Photo f drying chisel into pin blank.
Grip the pin blank in a vise and drive a chisel into the end grain, parallel to the growth rings.

Start by riving a straight-grained white-oak blank about twice as long as the needed pin, shearing off a piece just larger than the desired diameter (3⁄8") (photo, above.)

Photo of tapering the end of the pin.
Taper the end with a chisel or block plane so that the end fits into the dowel former. The tapered end also guides the finished pin through the drawbore.

Then, roughly round and taper the blank (photo, above.) Place the dowel former over a doghole in your bench, then drive the blank through (photo, below.)

Photo of hammer striking pin
Hammer the blank through the dowel former. The end will likely mushroom and possibly split. Use the best section for your drawbore pin.
Photo of pin being driven into joint
You'll hear the dowel pin bottom out with a low-toned thud, versus a hollow high-toned "pang" as it's being driven.

To assemble the joint, I spread glue within the mortise and on the tenon, squeeze a bead of glue into the drawbore hole, and lightly coat the pin before driving it in. As you drive the pin through, you'll see the tenon being drawn tightly to the mouth of the mortise (photo, above.) Trim the excess (photo, below), sand the surfaces smooth, and you'll have a mechanically reinforced joint, beautifully accentuated with a contrasting wood.  

Photo of painters tape on back of flush saw
A piece of painter's tape on the back of a flush-trim saw lifts the blade just enough to prevent the teeth of the saw from marring your workpiece.

Source: Veritas dowel former no. 05J6320, Lee Valley, 800-871-8158, leevalley.com.